Nonequilibrium reacting flows: First principles based modeling for chemical kinetics and hydrodynamics

Predicting state of the gas hitting vehicles flying at hypersonic speeds (Mach ~5) is challenging and is an exciting area of research. Hypersonic flows create shock waves, which compress and heat the surrounding gas to high-temperatures, nearly thousands of Kelvins. At these high temperatures, air molecules (nitrogen and oxygen) dissociate into atomic species. Predicting the extent of dissociation and recombination of atomic species is important since the state of the gas near the vehicle surface determines heating rates and gas-surface chemistry that damages the heat shield. Since experiments in ground test facilities do not mimic such extreme flight conditions, numerical simulation plays an important role. Predictive numerical simulations require accurate reaction chemistry models. Computational models developed thus far range from simple empirical models fit to limited experimental data to models with millions of input parameters that track individual quantized energy state transitions. The level of model fidelity required for accurate engineering analysis remains an open question of active research. Models coupling internal energy and dissociation chemistry tend to be developed at either the kinetic scale or the continuum scale. In this work, we develop new nonequilibrium models for shock heated flows that are analytically consistent between kinetic and continuum scales and are based on recent ab-initio data, applicable to large-scale CFD and direct simulation Monte Catlo (DSMC) simulations.

Nonequilibrium Hydrodynamics: The Navier-Stokes equations, typically employed even at strong non-equilibrium conditions, wherein thermodynamic fluxes such as stresses and heat flux vector are based on linear irreversible thermodynamics, not be accurate in multiscale and multiphysics scenarios encountered in hypersonic flows. Similarly, the Navier-Stokes equations are known to breakdown in rarefied (low density) gas flows. Therefore, a new formalism is proposed to circumvent these issues, which can also benefit, hybrid methods that can combine continuum description using the Navier-Stokes equations and microscopic description, necessary for efficient high-fidelity numerical simulations. Other wide range of physics problems such as nano-scale flows, plasma physics modeling, and general complex gas flows can also benefit from the proposed new non-equilibrium hydrodynamic formalism.

Speaker: Dr. Narendra Singh

Biography: Dr. Narendra Singh graduated with a PhD (and MS) in Aerospace Engineering (with minor in Mathematics and Chemistry) from University of Minnesota. Narendra obtained his undergraduate degree (with Honors) in MechE from IIT Bombay. In his doctoral thesis, Narendra developed chemical kinetics models for DSMC and CFD using first principles-based approach. In addition, Narendra (along with Prof.Agrawal) has developed higher order equations for rarefied and strong nonequilibrium flows, known as O-13 and O-Burnett equations, where O ‘refers’ to Onsager due to the consistency of equations with Onsager’s reciprocity principle. Narendra Singh did his 2 years postdoc in MechE at Stanford, where his research spanned particle-laden flows, carbon sequestration, and ultrafast chemistry at SLAC. Currently, he is a postdoc research associate at Center for Hypersonic, UIUC, and developing reduced order models for chemical kinetics.